1. Field of the Invention
The present invention relates in general to magnetic separation and, in particular, to improved methods and apparatus for making continuous magnetic separations of flowable mixtures of particles of different magnetic susceptibilities.
2. The Prior Art
Heretofore magnetic separators have fallen generally into two classes, those which employ a katadynamic magnetic field, i.e., a field in which the magnetic force exerted on a paramagnetic particle (as used herein including ferromagnetic and ferrimagnetic particles) increases in the direction of increasing magnetic field strength, and those which employ an isodynamic magnetic field, i.e., a field in which the force exerted on a paramagnetic particle is substantially constant throughout the working area of the field. The prior art also discloses an anadynamic magnetic field, i.e., a field in which the magnetic force exerted on a paramagnetic particle decreases in the direction of increasing field strength. Uses of the anadynamic field for magnetic separation have not been broadly developed. Typically, katadynamic magnetic separators make separations by attracting particles towards the magnetic poles or by otherwise deflecting the particles from their original path of travel through the separator. They have the advantage that many pole piece configurations may be used, thereby allowing flexibility in the design and arrangement of the pole pieces to suit particular applications, and of allowing control of the magnetic force exerted on the particles to be separated by adjustment of the spacing between the pole pieces or the applied magnetic field intensity, or both. Separators of this type, however, have either brought the particles into contact with the poles, thus presenting a cleaning problem among others, or have required the use of moving elements, such as belts, discs, or the like, or a moving fluid stream to intercept particles attracted or deflected by the field and carry them out of the working area of the field for collection. In addition, prior katadynamic separators have largely been limited to the separation of ferromagnetic and paramagnetic particles. In the main, they have not been successfully applied to the separation of diamagnetic materials.
The first magnetic separator to employ an isodynamic working field was disclosed by S. G. Frantz and patented by him in U.S. Pat. No. 2,056,426, issued on Oct. 6, 1936 and assigned to the assignee of the present application. Such separators are manufactured by the S. G. Frantz Company, Inc., Trenton, New Jersey, under the trademark ISODYNAMIC. The Frantz ISODYNAMIC magnetic separator affords advantages relative to the katadynamic magnetic separator in that it permits precise separations according to magnetic susceptabilities of both paramagnetic and diamagnetic materials. It also enables separations to be made continuously and without bringing the particles into contact with the pole pieces or without requiring the use of mechanical elements or a moving fluid stream to intercept and remove the particles from the magnetic field. Unlike the katadynamic separator, however, the ISODYNAMIC separator requires specially shaped pole pieces to establish the isodynamic field. The configuration and separation of pole pieces, once determined, cannot readily be varied to allow adjustment of the magnetic force exerted on the particles without loss of the isodynamic character of the field.
The present invention is directed to the provision of improved magnetic separation methods and apparatus which combine in a unique way the advantages and capabilities of katadynamic, isodynamic, and anadynamic magnetic fields.